Matthew Muller, the notorious kidnapper whose most infamous crime was detailed in the Netflix documentary “American Nightmare,” appeared Friday in court and pleaded guilty to two additional crimes.

Wearing a brown Santa Clara County jail uniform, Muller, 47, replied with a taciturn series of “yes” answers as Superior Court Judge Cynthia A. Sevely confirmed he was admitting guilt in two home invasions in 2009. In both cases, Muller broke into homes in the early hours, bound his female victims and attempted to drug and sexually assault them.

In total, Muller is now suspected or convicted in at least six violent crimes, beginning when he was 16.

“This extremely dangerous person left a trail of traumatized and terrified victims,” Dist. Atty. Jeff Rosen said. “It took the collective courage of his victims and determined law enforcement officers to stop him. This nightmare is over.”

The Santa Clara charges against Muller came about as a result of the work of an unlikely team of law enforcement officers and the two victims in the Vallejo case, Denise Huskins and Aaron Quinn. Over the last 10 months, the pair said, they obtained clues about the crimes — and even confessions — from Muller before approaching local authorities with jurisdiction in the incidents.

“We knew there was more to this from the beginning, and clearly how things were handled from the beginning led to a lot of errors,” Huskins said in an interview last week. “We didn’t really have anyone in law enforcement that we trusted and we felt were doing this case justice.”

The first Santa Clara County incident took place on Sept. 29, 2009, when a Mountain View woman in her 30s told police she awoke to find a man on top of her. According to a description of the case from the Santa Clara County district attorney’s office, Muller demanded she drink a medicated beverage, then tied her up and said he was going to rape her.

The woman was able to persuade him to stop his assault, according to the district attorney’s office. Before leaving, Muller allegedly told her that she should get a dog for protection.

About three weeks later, on Oct. 18, 2009, a woman in nearby Palo Alto awoke to find Muller on top of her, according to the district attorney’s office. He tied the woman up and forced her to drink Nyquil.

Again, the woman was able to persuade him to stop, according to prosecutors. And again, before leaving, he gave the woman “crime prevention advice,” according to the district attorney’s office.

Last week, Muller was also charged in another new case, in the Contra Costa town of San Ramon, after authorities examined evidence brought to light through Huskins’ and Quinn’s investigative efforts.

After “American Nightmare” came out, Huskins and Quinn were contacted by an unlikely ally: the police chief in the Monterey Bay town of Seaside, Nick Borges. He had seen the documentary and wanted to help.

That Borges had nothing to do with the case didn’t stop him from becoming involved. He invited Huskins and Quinn to speak to law enforcement in Seaside to share their belief that police interrogation methods that focused on Quinn’s guilt had sent the investigation down the wrong road.

Borges also persuaded the detective ultimately responsible for Muller’s arrest, Misty Carausu, to come.

The four met with El Dorado County Dist. Atty. Vern Pierson, who has jurisdiction in the county where Huskins was held captive — and the seeds of a new investigation were planted.

At lunch after the law enforcement conference, Huskins and Quinn told Borges about their frustrations, and a desire to reach out to Muller personally to seek answers. But the couple feared that could present risks. Borges offered to write Muller on their behalf.

Muller wrote back, giving details of other crimes and even legal declarations with confessions.

Armed with the new information, Pierson, who had been working with the FBI and other agencies, in November traveled to Tucson to interview Muller in person. Over two days, according to Pierson, Muller shared more details, including information on a Northern California attack he claimed to have committed when he was 16. That case is still under investigation, Pierson said.

In Huskins’ case, which the Netflix documentary is based on, Muller broke into her Vallejo home in March 2015 and drugged and bound her and her then-boyfriend, Aaron Quinn. Muller blindfolded them with swim goggles and gave them medicine to make them sleepy. He put headphones on Quinn and played recordings designed to make Quinn think he was dealing with more than one kidnapper.

Muller then put Huskins into Quinn’s car and drove off with her, eventually taking her to his family’s cabin in South Lake Tahoe. He held her there for two days and sexually assaulted her, before driving her across California and releasing her in Huntington Beach.

Initially, Vallejo police dismissed Quinn’s account of his girlfriend being spirited away by a kidnapper — or kidnappers — who put headphones on him and made him drink a substance that made him sleepy. Officers interrogated Quinn for hours, brushing aside his story and theorizing he was behind her disappearance.

When Huskins turned up, police grew more suspicious, questioning how a kidnapping victim could reappear hundreds of miles away wearing sunglasses and carrying an overnight bag.

Huskins “did not act like a kidnapping victim,” retired Vallejo Police Capt. James O’Connell later said in a sworn statement.

Police tried to get Huskins and Quinn to turn on each other and admit there had been no crime, offering immunity to whoever flipped first, according to statements from their family members.

Then, police went public with that sentiment. “There is no evidence to support the claims that this was a stranger abduction or an abduction at all,” Police Lt. Kenny Park said in a statement at the time. “Given the facts that have been presented thus far, this event appears to be an orchestrated event and not a crime.”

However, less than three months later, evidence gathered from a June 5, 2015, home invasion robbery in the Bay Area community of Dublin helped authorities link Muller to the kidnapping. That case led authorities and Carausu, the detective, to the Muller family’s South Lake Tahoe cabin, where they found, among other things, Quinn’s computer, goggles and tape with a strand of long blond hair.

Huskins and Quinn, who later married, sued the Vallejo Police Department for defamation and reached a $2.5-million settlement in 2018.

Muller, a Harvard-educated lawyer and former Marine, pleaded guilty in 2016 to kidnapping Huskins. In 2022, he pleaded guilty to additional charges of sexually assaulting her. Until he was transported to Santa Clara County to face the new charges, he was serving his 40-year sentence at a federal prison in Tucson.

Muller is expected to return to Santa Clara County Superior Court on Feb. 21 for sentencing.